Some gainers argue that the physical dangers of obesity are greatly exaggerated—that it's possible to be both fat and healthy. Jorge Rivera sees his doctor twice a year and says he'll stop growing if his health ever deteriorates. He says his cholesterol is 211, at the borderline-risk level, but his blood pressure is an optimal 119/74. Yet his body-mass index is 45—a dangerously high figure on a scale where 30 indicates obesity. A 2003 Johns Hopkins study calculates that a BMI that high means his life may end 13 years sooner than it would if he were at a normal weight.

If Rivera is worried, he doesn't show it. "I've had a good run," he says with a shrug. "People leave comments on YouTube telling me I'm going to die. And I'm like, 'You're gonna live forever?'"